Add cut potatoes into a large pot and cover with cold water.
Cook over medium heat and simmer potatoes for 15 to 20 minutes or until fork tender.
In a deep pot (you can use the one you used for the potatoes) add butter, garlic and rosemary sprigs.
Cook over medium-low heat until butter starts to brown.
Remove from heat.
Remove rosemary sprigs and discard.
Add potatoes to the prepared butter.
Season with salt and pepper. Depending on the saltiness of the parmesan cheese you are using, you may want to wait to salt the potatoes until the end.
Add parmesan cheese and sour cream; mash using a potato masher until smooth and creamy.
Taste for salt and pepper and adjust accordingly.
If you like the potatoes creamier, add more sour cream.